In any electrical power network it is desirable to attempt to predict the effects on the system of particular contingencies, such as a given power line being interrupted, so that the proper response to that contingency can be planned ahead of time and acted upon quickly in the event of such contingency. For example, where a major power line is interrupted, by taking action to improve stability, such as high response static exciters, braking resistor, shedding generation capacity and high speed single and three pole reclosing, an entire system failure may be avoided.
In some systems, generation shedding has proven to be one of the most effective discrete supplementary controls for maintaining stability. For every foreseeable disturbance, the amount of generation shedding is calculated and provided to the system operators in a look-up table format. With these tables, the operators preset the generator shedding controls for every major element outage. In the past, these tables have been prepared using off-line calculations. However in real system operation, the conditions frequently do not match those studied off-line, so the guidelines produced are provided on the conservative side. Operating power system networks on such conservative guidelines can have significant cost implications. Therefore it is desirable to provide an effective on-line transient stability method which can assess the dynamic security limits of a power system (the maximum transfer limit or the minimum required remedial action for the system to remain stable) for those contingencies likely to cause dynamic security violations for the operating conditions encountered in real-time.
The North American Reliability Council sets criteria for system dynamic security which require the simulation of complex switching sequence of various equipment in a system at different locations following a fault, based on the actual system structure. Such series of simulated events in such simulations, which simulate possible real contingencies, are referred to herein as the study contingencies, or natural contingencies, as contrasted to artificial or unnatural contingencies.